Rails indexes is a small package of 2 rake tasks that scan your application models and displays a list of columns that probably should be indexed.

Display a migration for adding/removing all necessary indexes based on associations:

rake db:show_me_a_migration

UPD: Another similar plugin ambitious_query_indexer

Ambitious Query Indexer is a Rails plugin to identify database indexes that are missing. Rather than looking at tables and guessing what needs indexing, it will locate and run as many queries as it can find and suggest indexes that could be added based upon its finding.